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42 lines
1.4 KiB
Go
42 lines
1.4 KiB
Go
package header
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import "netstack/tcpip"
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// Checksum 校验和的计算
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// UDP 检验和的计算方法是： 按每 16 位求和得出一个 32 位的数；
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// 如果这个 32 位的数，高 16 位不为 0，则高 16 位加低 16 位再得到一个 32 位的数；
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// 重复第 2 步直到高 16 位为 0，将低 16 位取反，得到校验和。
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func Checksum(buf []byte, initial uint16) uint16 {
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v := uint32(initial)
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l := len(buf)
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if l&1 != 0 {
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l--
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v += uint32(buf[l]) << 8
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}
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for i := 0; i < l; i += 2 {
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v += (uint32(buf[i]) << 8) + uint32(buf[i+1])
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}
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return ChecksumCombine(uint16(v), uint16(v>>16))
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}
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// ChecksumCombine combines the two uint16 to form their checksum. This is done
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// by adding them and the carry.
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func ChecksumCombine(a, b uint16) uint16 {
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v := uint32(a) + uint32(b)
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return uint16(v + v>>16)
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}
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// PseudoHeaderChecksum calculates the pseudo-header checksum for the
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// given destination protocol and network address, ignoring the length
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// field. Pseudo-headers are needed by transport layers when calculating
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// their own checksum.
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// hash(protocol, hash(dst, hash(src, 0)))
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func PseudoHeaderChecksum(protocol tcpip.TransportProtocolNumber, srcAddr tcpip.Address, dstAddr tcpip.Address) uint16 {
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xsum := Checksum([]byte(srcAddr), 0)
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xsum = Checksum([]byte(dstAddr), xsum)
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return Checksum([]byte{0, uint8(protocol)}, xsum)
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}
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